An ongoing report on S366:Evolution

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Tony Arnold and Bill Parker compiled what may be the largest, most complete set of data on the evolutionary history of any group of organisms, marine or otherwise. The two scientists amassed something that their land-based colleagues only dreamed about: An intact fossil record with no missing links.

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"It's all here--a virtually complete evolutionary record," says Arnold. "There are other good examples, but this is by far the best. We're seeing the whole picture of how this group of organisms has changed throughout most of its existence on Earth."

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The organism that Arnold and Parker study is a single-celled, microscopic animal belonging to the Foraminiferida, an order of hard-shelled, planktonic marine protozoans. Often shortened to "forams," the name comes from the Latin word foramen, or "opening." The organisms can be likened to amoebas wearing shells, with perforations through which their protoplasm extends. The foram shell shapes range from plain to bizarre.
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"There's a nifty passage in Darwin," says Arnold, "in which he descirbes the fossil record as a library with only a few books, and each book has only a few chapters. The chapters have only a few words, and the words are missing letters."

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"Well, in this case, we've got a relatively complete library," says Arnold. "The 'books' are in excellent shape. You can see every page, every word."

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As he speaks, Arnold shows a series of microphotographs, depicting the evolutionary change wrought on a single foram species. "This is the same organism, as it existed through 500,000 years," he says. "We've got hundreds of examples like this, complete life and evolutionary histories for dozens of species."

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About 330 species of living and extinct planktonic forams have been classified so far. After thorough examinations of marine sediments collected from around the world, micropaleontologists now suspect these are just about all the free-floating forams that ever existed.

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The species collection also is exceptionally well-preserved, which accounts largely for the excitement shared by Parker and Arnold. "Most fossils, particularly those of the vertebrates, are fragmented--just odds and ends," says Parker. "But these fossils are almost perfectly preserved, despite being millions of years old."
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Darwin termed the process gradualism, a theory that invokes the slow accumulation of small evolutionary changes over a large period of time, as a result of the pressures of natural selection. What Arnold and Parker found is almost a textbook example of gradualism at work.

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We've literally seen hundreds of speciation events," syas Arnold. "This allows us to check for patterns, to determine what exactly is going on. We can quickly tell whether something is a recurring phenomenon--a pattern--or whether it's just an anomally. This way, we cannot only look for the same things that have been observed in living organisms, but we can see just how often these things really happen in the environment over an enormous period of time.

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